This invention relates to a process for removing metals from contaminated materials such as soils, sediments, sludges, and aquatic environments, so that the contaminated material may be reclaimed.
Radionuclides and toxic metals are present in certain wastes, particularly in waste disposal sites such as those of the Department of Energy. These contaminants may also leach from the waste sites and cause environmental contamination. For decontamination of waste material, both the metal and radionuclide contaminants must be removed from the contaminated site. This presents the problems of removing radionuclides and metals from the soil or water so that the site can be returned to a useful condition. It would be desirable and beneficial to the environment to provide a comprehensive method for the removal of toxic metals and radionuclides from contaminated sites with reclamation of the soil and water.
Previous large scale methods devised to deal with the problem of contaminated soil have utilized caustic reagents such as hot sulfuric or hydrochloric acids and oxidizing agents such as sodium hypochlorite to extract metals from soil. While these methods can remove contaminants, they also cause irreparable damage to the soil. Prior art methods generate secondary waste streams which create additional hazardous waste disposal problems. Various soil washing methods were discussed by C. W. Francis in a presentation, "An Assessment of Soil Washing to Remove Uranium and Mercury From Oak Ridge Soils", at the Department of Energy's Soil Washing Workshop, Aug. 28-29, 1990. See also, B. E. Campbell and S. S. Koegler, "In Situ Vitrification of a Mixed Radioactive and Hazardous Waste Site", Energy Research Abstracts 7186 (March 1991) describing the use of electrodes inserted into the ground to vitrify a waste site; C. Madic et al., "Extraction of Metal Ions by Neutral B-Diphosphoramides", Energy Research Abstracts 3636 (February 1991) describing the use of bidentate phosphoramides enhanced by nitric acid to extract metal ions such as lanthanides, uranyl and the transuranium elements AM(III) and Pu(IV); K. H. Kim, et al., "Immobilization of Radioactive Strontium in Contaminated Soils by Phosphate Treatment", Energy Research Abstracts 1993 (January 1991) describing coprecipitation of strontium 90 with Ca-, Al- and Fe-phosphate in contaminated soils; R. Raghaven, et al. "Cleaning Excavated Soil Using Extraction Agents: A State-of-the-Art Review. Final Report June 1985-January 1989", Energy Research Abstracts 5106 (Jan. 15, 1990) describing three generic types of extractive treatments for cleaning excavated soils: water washing augmented with a basic or surfactant agent to remove organics and with an acidic or chelating agent to remove organics and heavy metals, organics-solvent washing to remove hydrophobic organics and polychlorinated biphenyls, and air or steam stripping to remove volatile organics; R. L. Kochen and J. D. Navratil, "Americium and Plutonium Removal From Contaminated Soil", Energy Research Abstract 36247 (Sep. 15, 1985) describing reducing the volume of contaminated soils by wet-screening, attrition scrubbing, wet-screening with additives, and fixation by conversion to glass.
Various specialized kinds of metal removal from soil have been known in the art. For example, a method for solubilizing a metal-containing precipitate has been described in U.S. Pat. No. 4,973,201 to Paul et al.. This patent describes a method for solubilizing precipitated alkaline earth metal sulfate scale in contaminated earth by contacting the earth with a polyaminocarboxylic acid chelating agent (ETDA or DTPA) and an oxalate ion synergist and leaching the solubilized precipitate from the earth with water. To dispose of the dissolved sulfates, the leachate is said to be treated by chemical methods or returned to a subterranean formation.
In a study directed to devising a method to correlate with Pu uptake by plants in contaminated soils, the extraction of plutonium from contaminated soil with inorganic and organic compounds including citrate was described by H. Nishita et al., "Effect of Inorganic and Organic Compounds on the Extractability of .sup.239 pU from an Artificially Contaminated Soil", J. Environ. Qual. 6, 451-455 (1977).
Radioactive contaminated components of nuclear reactors have been decontaminated using citric acid or citrates as described, for example, in U.S. Pat. Nos. 4,839,100, 4,729,855, 4,460,500, 4,587,043, 4,537,666, 3,664,870 and 3,103,909. In these patents, metal recovery methods involve ion exchange columns, porous DC electrodes or combusting the organics.
U.S. Pat. No. 4,943,357 describes the use of ultra violet light with a wavelength less than 210 nm using a mercury lamp for photodegradation of metallic chelate complexes, particularly nickel ethylenediaminetetraacetic acid (EDTA) from nickel plating baths. An article by A. Adams and T. D. Smith, "The Formation and Photochemical Oxidation of Uranium(IV) Citrate Complexes", J. Chem. Soc. 4, 4846-4850 (1960) describes photochemical oxidation of uranium(IV) chelate of citric acid using a tungsten filament lamp.
The effects of chemical speciation on microbial mineralization of metal organic complexes by sewage microorganisms Pseudomonas alcaligenes, Pseudomonas pseudoalcaligenes and Listeria sp. is described by E. L. Madsen and M. Alexander, "Effects of Chemical Speciation on the Mineralization of Organic Compounds by Microorganisms", Applied and Environmental Microbiology 50, 342-349 (1985). L. Brynhildsen and T. Rosswall, "Effects of Copper, Magnesium, and Zinc on the Decomposition of Citrate by a Klebsiella sp.", Applied and Environmental Microbiology 55, 1375-1379 (1989) describe the effects of Cd.sup.2+, Cu.sup.2+, Mg.sup.2+ and Zn.sup.2+ on the decomposition of citric acid by a Klebsiella sp. also isolated from sewage. These researchers found that some metals may render citric acid resistant to bacterial degradation.
Many chelates have been shown to be either poorly biodegraded by microorganisms under aerobic conditions or to undergo little biodegradation under anaerobic conditions. See, e.g. Francis, A. J., "Microbial Dissolution and Stabilization of Toxic Metals and Radionuclides in Mixed Wastes", Experientia 46, 840-849 (1990); and A. J. Francis et al., "Microbial Transformations of Uranium in Wastes", Radiochemica Acta 52/53, 311-316 (1991).
Various isolated concepts have been described in the art. But the problem of providing thorough waste site decontamination heretofore has not been solved. Furthermore, nothing has suggested a total method for reclaiming radionuclide or toxic metal-contaminated soil, sediments, sludges and water with recovery of the contaminating metals to reduce toxic waste, and with restoration of undamaged soil or water.
Accordingly, it is an object of the invention to provide a comprehensive method which can remove even recalcitrant radionuclides from contaminated soil, sediments, sludges and water.